Conventionally, the image forming apparatus for forming the image with the liquid developer containing toner and a carrier liquid has been proposed. In the image forming apparatus, the liquid developer accommodated in a mixer is supplied to a developing device and is subjected to development (Japanese Laid-Open Patent Application 2001-201943). A toner concentration of the liquid developer in the mixer is adjusted on the basis of a detection result of a concentration sensor and a liquid amount of the liquid developer in the mixer is adjusted on the basis of a detection result of a liquid amount sensor, respectively. The toner or a high-concentration liquid developer (these are referred to as a supply agent) is supplied from a toner tank to the mixer in the case where the toner concentration of the liquid developer is lower than a predetermined target value. On the other hand, the carrier liquid is supplied from a carrier tank to the mixer in the case where the liquid amount of the liquid developer is less than a lower limit or in the case where the toner concentration of the liquid developer is higher than the predetermined target value. The supply agent and the carrier liquid which were supplied to the mixer are mixed with an already-existing liquid developer by the mixer.
Incidentally, in the case where the liquid amount of the liquid developer is less than the predetermined lower limit, when the carrier liquid is only supplied, the toner concentration after supply of the carrier liquid lowers. Therefore, simultaneously with the supply of the carrier liquid for liquid amount adjustment, the supply agent in an amount depending on a supply amount of the carrier liquid is automatically supplied so that the toner concentration is unchanged before and after the supply of the carrier liquid (this is referred to as a non-interacting function). The supply amounts of such supply agent and carrier liquid are controlled by changing operation times of a pump for supplying the supply agent from the toner tank to the mixer and a pump for supplying the carrier liquid from the carrier tank to the mixer.